1. Field of the Invention
The present invention relates generally to technology for increasing the transmission capacity of a Mobile Station (MS) in a distributed antenna system using multiple distributed antennas that are connected to a Base Station (BS) by wire or a dedicated line, and more particularly to an apparatus and method for simultaneously transmitting signals to multiple MSs by using a Maximal Ratio Transmission (MRT) scheme.
2. Description of the Related Art
With the growth of the information industry, there is an increasing need for technologies that enable various types of mass data to be transmitted at high speed. As a result, research is actively being conducted on a multi-hop scheme that is designed in an attempt to reduce shadow areas and extend coverage by disposing multiple distributed antennas in existing cells.
A multi-hop scheme is basically divided into a scheme in which a BS and Relay Stations (RSs) are wirelessly interconnected, and a scheme in which a BS and RSs are connected by wires. In the wireless multi-hop scheme, transmission power is reduced because multiple wireless RSs partially sharing wireless resources with a BS reduce a transmission range as compared to a conventional cellular system only including a BS. Also, path loss is reduced due to shortened transmission distances between the RSs and the MSs, which enables high-speed data transmission and increases the transmission capacity of a cellular system.
However, because transmission for data relay is additionally required as compared to a single-hop scheme, and many RSs share the limited resources of the system, deterioration of service quality is often caused. That is, while the multi-hop scheme using wireless RSs may improve the Signal-to-Interference plus Noise Ratio (SINR) of an MS in the cell edge area, it also has difficulty in largely extending system capacity because frame resources are partially used for relay transmission, and thus effective channel resources available to the MS are then reduced.
The wired multi-hop scheme, in which a cell is configured by linking a BS and each RS by an optical cable, has a basic network configuration that is similar to that of the multi-hop scheme using wireless RSs, but is different therefrom in that the link between the BS and each RS is a wired link, and fixed RSs are used.
In comparison to the wireless multi-hop scheme, the wired multi-hop scheme is less cost efficient because optical cables must be installed, and is less convenient as the RSs are difficult to move after installation. However, the wired multi-hop scheme is advantageous over the wireless multi-hop scheme in that there is no signal loss between a BS and each RS, and interference is reduced. Also, by transmitting various types of control signals over a wired link between a BS and each RS, it is possible to apply high-complex and high-efficient resource allocation techniques and collaborative signal transmission schemes between the RSs.
Mobile cellular systems that currently operate in the wired multi-hop scheme provide services to multiple MSs in a cell by using Multiple Access (MA) schemes, such as Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), and Frequency Division Multiple Access (FDMA). However, because the MA schemes divide and use a limited amount of available resources, such as codes, time, and frequencies, they do not often satisfy the demands for high-speed transmission of mass multimedia information to more users with the growth of mobile communication technology.
As a solution to this problem, research is actively conducted on a scheme for simultaneously transmitting signals to multiple MSs by using Multiple-Input Multiple-Output (MIMO) technology in which signals are transmitted through multiple antennas disposed at transmitting and receiving ends. Examples of such schemes using MIMO technology include a Dirty Paper Coding (DPC) scheme and a beamforming scheme.
The DPC scheme, which is essentially an interference signal pre-cancellation technique, is a scheme in which an interference signal known to a transmitting end is pre-cancelled at the transmitting end, such that the transmitting end is not affected by the interference signal. Although the DPC scheme is considered a scheme that provides the most optimal performance from among currently known schemes, it is difficult to actually apply because the complexity of encoding and decoding increases as the number of MSs included in the system increases.
In the beamforming scheme, which is smart antenna schemes, a beam of a transmit/receive antenna is limited to a corresponding MS. When signals are transmitted using this scheme, the resulting increase in transmission capacity is lower than that of the DPC scheme, but implementation complexity also comparatively reduced. Accordingly, the beamforming scheme is often referred to as a quasi-optimal technique.
Additionally, the beamforming scheme may simultaneously transmit signals to multiple MSs by multiplying signals for respective MSs by beamforming vectors independent of each other and transmitting the resultant signals. However, there is a problem in that when the beamforming scheme is applied, there may be interference between MSs receiving the signals, depending on channel characteristics in the actual environment, which results in performance deterioration. Therefore, there is still a need to minimize interference between MS signals, while increasing the overall transmission capacity of a cell in a distributed antenna system.